This invention relates to an intrusion detection system for providing an alarm responsive to intrusion upon residential or industrial premises.
In present-day electronic security systems, there are a wide variety of intrusion-detecting transducers. For example, magnetic switch transducers are used to detect the opening or closing of windows and doors. There are also ultrasonic motion detectors and vibration sensors, as well as other detecting devices. With all of these transducers or sensors, some provision is made to provide an alarm after the system is set (armed) and an intrusion takes place. Most known security systems require a control station from which the individual intrusion system can be armed and monitored, and additionally require the use of wiring from such control station to the sensors in the system.
Many intrusion detection systems utilize normally-closed magnetic switches on the windows and doors. Such switches include a magnet portion and a contact portion. When a double-hung window or a door is opened, the magnet portion is moved away from its associated contact portion, and an alarm is sounded. When magnetic switches are used on windows, a significant disadvantage results in that the window may be broken, all of the glass removed, and an intruder may enter without producing an alarm condition. This disadvantage occurs since the magnetic switch can only be operated when the double-hung window or the door is physically displaced to separate the magnet from the associated contacts.
Systems have been designed which utilize a vibration sensor for producing an alarm. When such systems have been mounted on windows and doors, a significant number of false alarms have occurred due to the presence of environmental vibrations not associated with the unauthorized intrusion into a residence or an industrial enclosure. In order to reduce the number of false alarms generated with vibration sensors, existing equipment must be set to operate at a reduced sensitivity level. However such lowering of the sensitivity level makes the system considerably more susceptible to defeat by an intruder who does not produce vibrations of an amplitude requisite to trigger the lowered-sensitivity equipment during entry to the guarded premises.
It is therefore a primary object of the present invention to provide an intrusion detection system particularly useful with a vibration sensor, and which does not require reduction of the system's sensitivity to minimize false alarms generated by spurious vibrations.
Self-contained intrusion detection systems have previously been designed for mounting directly to doors and/or windows. These systems typically have a key-operated power switch, to prevent the intruder from disconnecting the power after the system has detected the intruder's presence and has begun to produce an alarm signal. Use of the key-operated switch increases the cost of the equipment, and has a further disadvantage in that the key may be lost or misplaced.
Another important object of the invention is to provide an intrusion detection system which initiates the alarm signal upon intrusion, and thereafter maintains the alarm signal for a present minimum time period even if the system's power switch is displaced to its "off" position.